Designs for Switching "Domains" in 1-to-1 conversations
In these mockups, for lack of a better term and for simplicity's sake, I've dubbed the account/address/number/ID you send messages from and likewise those of the recipient(s) of your messages a "domain".
Domain Switching
The goal for this design is that in a conversation you are having with a person, the process of switching the domain you are sending from and the one they are receiving messages on should be seamless and that the application preserves conversation continuity and the context of the messages.
The application should also be informative of these changes in the conversation view with inline alerts but not be too disruptive of the flow of conversation, so their styling should remain subtle.
All actions for changing the domain you are sending from should be tied to message composition and appear in a new submenu on the bottom toolbar, and all actions for changing the recipient's domain should be tied to title menu attached to their contact information at the top of the window.
Contact Cards
These concepts presume well-managed contact cards that serve as the basis for the items in the menus for switching between their domains and the items in both your and your contact's menus for switching domains will be pulled from the relevant contact cards in your addressbook. The caveat here is that if a contact's domains are spread across different cards the application won't know that they are available to be changed to as it would appear as if they are different contacts/people.
Multiple Account Settings
Additionally, the design for multiple account settings has been adjusted to account for these new features largely to accommodate the ability to title and colour your multiple accounts.